1. Field of the Invention
The present invention relates to a speed control circuit for a dc brushless motor and more particularly to a switch circuit and a voltage-detection circuit for changing speeds of the dc brushless motor in response to changes in voltage of the power supply.
2. Description of the Related Art
Currently, fans are used for forced air cooling of the electronic devices, such as power supply, CPU of a computer, display card, and computer components etc. Recently, there is a trend to adjustably operate fans at high or low speed depending upon ambient conditions. In high temperature ambiance, a fan must operate at high speed to reduce or remove heat produced by electronic components so as to ensure that these electronic components are operated normally and that the heat will not damage the electronic components. In low temperature ambiance, the fan must operate at low adequate speed for saving power and decreasing noise.
FIG. 1 illustrates a schematic view of a conventional driver circuit for a single phase dc brushless motor.
Referring to FIG. 1, the driver circuit for the single phase dc brushless motor includes a coil L1, a Hall element IC1, a driver element IC2, and a capacitor C1. The Hall element IC1 is used for detecting changes of a permanent magnetic field of a rotor to thereby output weaker Hall voltage. And then the Hall voltage is magnified by the driver circuit IC2 to further output alternative signals (square waveforms) from two terminals O1 and O2 to the bridging transistors Q1 through Q4. The coil L1 is actuated in turn to change current in direction so as to rotate the rotor.
FIG. 2 illustrates a schematic view of a conventional driver circuit for a double phase dc brushless motor.
Referring to FIG. 2, the driver circuit for the double phase dc brushless motor includes a first coil L1, a second coil L2, a capacitor C1, a Hall driver element IC3, a first resistor R1, a second resistor R2, and a transistor Q1. The resistors R1 and R2 and transistor Q1 form a rotational speed detective circuit. The Hall driver element IC3 is used for detecting changes of a permanent magnetic field of a rotor and then the Hall voltage is magnified to output alternative signals (square waveforms) from two terminals O1 and O2 to the coils L1 and L2 respectively. The coils L1 and L2 are actuated in turn to change current so as to rotate the rotor.
In a cooling fan system, there is generally provided two voltage levels to a driver circuit for various desired amounts of forced air. For example, a high voltage level of 12V controls a fan operating at 3000 rpm, and a low voltage level of 6V at 1500 rpm. Therefore, the cooling fan system operates at different speeds to provide cooling capacity depending upon the ambient conditions.
FIG. 3 illustrates a voltage diagram for speed of a conventional dc brushless motor. Between two specific voltages, ranging from 12V to 6V for example, are a high and a low speed. However, the voltage-to-speed curve is nonlinear. When 12V is input to the cooling fan system, the motor is operated at the high speed of 3000 rpm; but when the cooling fan system input is changed to 6V, the motor is operated at a low speed of 2000 rpm as a result of the non-linearity. Accordingly, the cooling fan system fails to certainly operate at a predetermined low speed of 1500 rpm to meet the system demand.
The motor structure and the nonlinear feature limit the conventional dc brushless motor whose speed cannot be adjusted depending upon the system needs. In order to improve these limitations of the conventional motor to perform desired speeds, it is necessary to be redesigned.
The present invention intends to provide a speed control circuit comprising a switch circuit and a voltage-detection circuit used for adjusting an operating voltage depending upon speeds of the motor. The operating voltage of desired speed of the motor is determined by the rated range of input voltage in such a way to mitigate and overcome the above problem.